· by Michael Blair, Nashville's Sunrooms
A screened porch that keeps out mosquitoes can still leave you scratching at dusk. The usual culprits are biting midges, the insects most people call no-see-ums, and they are small enough to get through the screen most porches are built with. Finer mesh stops them. How fine is where the screen maker and the extension service give different numbers, and every step finer costs some breeze. Here is what each one says, so the choice fits your porch rather than a catalog.
How small is a no-see-um?
Usually 1 to 2 millimeters long, small enough that ordinary screen does not stop it. The University of Florida’s EDIS fact sheet on biting midges puts most adults in that range and says many species are smaller than 1/24 inch. It also says regular window screens do not keep biting midges out of indoor spaces, and that the screen with openings small enough to do it is not what is normally used on windows and doors.
The University of Arizona’s No-See-Ums IPM sheet describes where they come from: near water, such as springs, ponds, lakes, river banks and swamps, and in towns, from mulch or sod that is watered too much. It says they are most active at dawn and dusk, which is exactly when a porch gets used.
What mesh size stops no-see-ums?
Something finer than standard porch screen, and the two numbers on offer are 20x20 and 30x30. The Arizona sheet says no-see-ums can get through standard 16x16 or 18x16 screen and recommends replacing it with 30x30 mesh wherever possible. Phifer, which makes porch screening, sells a 20x20 No-See-Ums mesh that it describes as designed for no-see-ums, sandflies, blackflies and midges, next to its standard 18x14 Pool and Patio screen.
Those are two different kinds of statement. Arizona’s is an extension recommendation; Phifer’s describes its own product, and its page gives no opening size or test result. We do not treat either number as settled. The point they share is that a screen meant for no-see-ums is a step finer than the screen most porches already have.
Does finer mesh cut the breeze?
Yes, some. More strands to the inch means smaller openings and more of the screen taken up by the strands themselves, so less air passes and the view out reads a shade darker. How much depends on the product, so look at a sample in person, ideally held up to a window on a breezy evening.
Width is the other practical point. Phifer lists its 20x20 No-See-Um mesh among the screens it makes in a 144-inch width, which matters on the wide openings of a porch. On a porch built to catch a west breeze with no water nearby, the trade may not be worth it; on one that looks out over a creek or a pond, it usually is.
Choosing screen for your porch
Choose by what bites and where the porch sits. Our screen rooms include small-mesh and pet-resistant screen options, and a porch that is already screened can usually be rescreened in a finer mesh without rebuilding the frame. We screen porches around Mt. Juliet and across the rest of Middle Tennessee. If bugs are only part of the problem, a screened porch or a sunroom compares the two. Ask us for an estimate and tell us when the biting starts.
- Mosquitoes and flies only, open lawn, no water close by: standard porch screen
- Bitten at dawn or dusk near a creek, pond or heavily watered beds: a no-see-um mesh, 20x20 as Phifer sells it or 30x30 as Arizona recommends
- A breezy porch where airflow matters most: compare samples side by side before committing
- A dog that leans on the screen: a pet-resistant screen on the lower panels
Sources
- University of Florida IFAS Extension, EDIS EENY-349 (IN626), Biting Midges, No-See-Ums, Culicoides spp. (2025) — Adults usually 1.0 to 2.0 mm, many species under 1/24 inch; regular window screens do not exclude biting midges; midge screen not typically used on windows or doors.
- University of Arizona Cooperative Extension, No-See-Ums IPM Short (2018) — No-see-ums pass through standard 16x16 or 18x16 screens, replace with 30x30 where possible; active at dawn and dusk; found near water and over-irrigated mulch or sod.
- Phifer, Pool and Patio Screening — 20x20 No-See-Ums mesh described as designed for no-see-ums, sandflies, blackflies and midges; 18x14 Pool and Patio screen; 144-inch widths. Manufacturer description, no opening size or test stated.
FAQ
No-See-Um Screen: Which Mesh Keeps Midges Out — questions
What is no-see-um screen?
It is screen woven finer than standard porch screen, so the openings are small enough to stop biting midges. Phifer sells one as a 20x20 mesh, while University of Arizona Extension recommends 30x30. Arizona Extension says midges, usually 1 to 2 millimeters long, get through standard 16x16 or 18x16 screen.
Will a screen porch keep out no-see-ums?
Not with ordinary screen. The University of Florida says regular window screens do not exclude biting midges, and Arizona Extension says they pass through standard 16x16 or 18x16 mesh. A porch screened with a finer no-see-um mesh, 20x20 or 30x30 depending on whose advice you follow, is what keeps them out.
Can no-see-ums get through window screens?
Yes. Biting midges are usually 1 to 2 millimeters long, and the University of Florida says regular window screens do not keep them out of indoor areas. The finer screen that does stop them is not normally used in windows or doors, so expect to see them indoors near water at dawn and dusk.
Is 20x20 mesh enough for no-see-ums?
The advice differs. Phifer sells its 20x20 mesh as designed for no-see-ums and other midges, while University of Arizona Extension recommends 30x30 mesh where standard screen is being replaced. Finer mesh passes less air, so weigh how bad the biting is against how much breeze the porch needs, and look at samples first.